1. Field
This invention relates generally to nuclear reactors, and more particularly, to nuclear reactors that employ top mounted control rods.
2. Related Art
The primary side of nuclear power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated and in heat exchange relationship with a secondary side for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes, for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump and the system of pipes which are connected to the vessel form a loop of the primary side.
For the purpose of illustration, FIG. 1 shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel 10 having a closure head 12 enclosing a nuclear core 14. A liquid reactor coolant, such as water, is pumped into the vessel 10 by pump 16, through the core 14 where heat energy is absorbed and is discharged to a heat exchanger 18, typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump 16, completing the primary loop. Typically, a plurality of the above-described loops are connected to a single reactor vessel 10 by reactor coolant piping 20.
An exemplary reactor design is shown in more detail in FIG. 2. In addition to the core 14 comprised of a plurality of parallel, vertical, co-extending fuel assemblies 22, for purposes of this description, the other vessel internal structures can be divided into the lower internals 24 and the upper internals 26. In conventional designs, the lower internals' function is to support, align and guide core components and instrumentation as well as direct flow within the vessel. The upper internals restrain or provide a secondary restraint for the fuel assemblies 22 (only two of which are shown for simplicity in FIG. 2), and support and guide instrumentation and components, such as control rods 28. In the exemplary reactor shown in FIG. 2, coolant enters the reactor vessel 10 through one or more inlet nozzles 30, flows down through an annulus between the reactor vessel and the core barrel 32, is turned 180° in the lower plenum 34, passes upwardly through a lower support plate 37 and a lower core plate 36 upon which the fuel assemblies 22 are seated and through and about the assemblies. In some designs, the lower support plate 37 and the lower core plate 36 are replaced by a single structure, the lower core support plate, at the same elevation as 37. The coolant flow through the core and surrounding area 38 in typically large, on the order of 400,000 gallons per minute at a velocity of approximately 20 feet per second. The resulting pressure drop and frictional forces tend to cause the fuel assemblies to rise, which movement is restrained by the upper internals, including a circular upper core plate 40. Coolant exiting the core 14 flows along the underside of the upper core plate and upwardly through a plurality of perforations 42. The coolant then flows upwardly and radially to one or more outlet nozzles 44.
The upper internals 26 can be supported from the vessel 10 or the vessel head 12 and include an upper support assembly 46. Loads are transmitted between the upper support assembly 46 and the upper core plate 40, primarily by a plurality of support columns 48. A support column is aligned above a selected fuel assembly 22 and perforations 42 in the upper core plate 40.
As will be explained in more detail hereafter, the reactor internals also include rectilinearly moveable control rods 28 for controlling the nuclear reaction within the core. The control rod assemblies, commonly known as rod cluster control mechanisms, typically include a drive shaft 50 and a spider assembly 52 of neutron poison rods that are guided through the upper internals 26 and into aligned fuel assemblies 22 by control rod guide tubes 54. The guide tubes are fixedly joined to the upper support assembly 46 and are connected by a split pin 56 force fit into the top of the upper core plate 40. The pin configuration provides for ease of guide tube assembly and replacement if ever necessary and assures that the core loads, particularly under seismic or other high loading accident conditions are taken primarily by the support columns 48 and not the guide tubes 54. This support arrangement assists in retarding guide tube deformation under accident conditions which could detrimentally affect control rod insertion capability.
FIG. 3 is an elevational view, represented in vertically shortened form, of a fuel assembly being generally designated by reference character 22. The fuel assembly 22 is the type used in a pressurized water reactor and has a structural skeleton which, at its lower end includes a bottom nozzle 58. The bottom nozzle 58 supports the fuel assembly 22 on the lower core plate 36 in the core region of the nuclear reactor. In addition to the bottom nozzle 58, the structural skeleton of the fuel assembly 22 also includes a top nozzle 62 at its upper end and a number of guide tubes or thimbles 54, which extend longitudinally between the bottom and top nozzles 58 and 62 and at opposite ends are rigidly attached thereto.
The fuel assembly 22 further includes a plurality of transverse grids 64 axially spaced along and mounted to the guide thimbles 54 (also referred to as guide tubes) and an organized array of elongated fuel rods 66 transversely spaced and supported by the grids 64. Although it cannot be seen in FIG. 3, the grids 64 are conventionally formed from orthogonal straps that are interleaved in an egg-crate pattern with the adjacent interface of four straps defining approximately square support cells through which the fuel rods 66 are supported in transversely spaced relationship with each other. In many conventional designs, springs and dimples are stamped into the opposing walls of the straps that form the support cells. The springs and dimples extends radially into the support cells and capture the fuel rods therebetween; exerting pressure on the fuel rod cladding to hold the rods in position. Also, the assembly 22 has an instrumentation tube 68 located in the center thereof that extends between and is mounted to the bottom and top nozzles 58 and 62. With such an arrangement of parts, fuel assembly 22 forms an integral unit capable of being conveniently handled without damaging the assembly of parts.
To control the fission process, a number of control rods 28 are reciprocally movable in the guide thimbles 55 located at predetermined positions in the fuel assembly 22. Specifically, a rod cluster control mechanism 80 positioned above the top nozzle 62 supports the control rods 28. The control mechanism 80 has an internally threaded cylindrical hub member 82 with a plurality of radially extending flukes or arms 52. Each arm 52 is interconnected to one or more control rods 28 (the arrangement of the central hub and radially extending flukes is also referred to as a spider mechanism), such that the control rod mechanism 80 is operable to move the control rods vertically in the guide thimbles 55 to thereby control the fission process in the fuel assembly 22 under the motive power of control rod drive shafts 50 which are coupled to the control rod hubs 80, all in a well-known manner. In the withdrawn position, the control rods are guided up into the control rod guide tubes 55 above the upper core plate 40 and in the fully inserted position the control rods occupy substantially the entire length of the guide thimbles 54 within the fuel assemblies as shown in FIG. 3. Alignment of the control rods through the upper internals 26 with the guide thimbles 55 in the fuel assemblies is maintained by guide cards 70 supported in a spaced tandem arrangement along the length of the control rod guide tubes 54.
FIG. 4 shows an enlarged view of the control rod assembly guide tube 54 shown between the upper support assembly 46 and the upper core plate 40 in FIG. 2. The guide tube 54 is made up of two sections, a lower guide tube section 78 and an upper guide tube section 84. The lower guide tube section 78 has a generally square cross section while the upper guide tube section 84 has a generally rounded cross section. The lower guide tube section 78 is joined to the upper guide tube section 84 at an intermediate coupling 86. The upper and lower guide tube sections 84 and 78 have a plurality of guide cards 70 supported in tandem in spaced relationship to each other along the length of the guide tube 54 with a continuous guided section 88 extending up from the bottom of the guide tube 54 a distance approximately equal to the spacing between the guide cards 70.
FIG. 5 is representative of the pattern of the openings in the continuous guided section 88, the guide cards 70 and the guide plate at the intermediate coupling 86, through which the control rod assembly 80 passes as it travels through the upper internals 26. The three-quarter round openings 72 guide the individual control rods 28 with the flukes 52 passing through the straight portions 74 connecting the circular openings 72 to the central opening 76 through which the hub 82 passes. The guide card illustrated in FIG. 5 is from the upper section 84 of the guide tube 54, but the pattern of the openings are representative of the opening pattern in the other guides as well; the difference being that the shape of the outer circumference changes from circular to generally square as one transitions from the upper section 84 to the lower section 78 of the guide tube 54.
Aggressive guide card wear has been observed at some operating nuclear plants. When the special guide plate at the intermediate coupling 86 is located within the series of allowable worn guide cards 70, the guide plate can be replaced during an outage to extend the life of the guide tube, in lieu of replacing the lower guide tube assembly 78, if heavily worn. This mitigation technique reduces schedule, costs and radioactive waste generated while enabling continued safe plant operation, albeit for a limited portion of the remaining life of the plant.
Accordingly, a more permanent fix for guide card wear is desired that can be achieved on a similar schedule to that required to replace the guide plate at the intermediate coupling 86.
Additionally, such a repair is desired that would not require the generation of additional radioactive waste and is substantially comparable in cost to replacement of the guide plate.